This invention relates to apparatus for testing the operation of relief valves, sometimes also called safety valves. Such valves are mounted on pressurized vessels, pipes, tanks and similar apparatus. The valves are designed to protect the pressurized apparatus by opening at a predetermined point above normal operating pressure. The pressure at which the valve opens is called the set point. Under ordinary conditions relief valves remain closed and are not actuated. As a result the set pressure may change over time. Sound management principles dictate that relief valves be checked periodically to assure proper operation of the valves, namely, that the set pressure remains at the designed value.
The present invention concerns testing apparatus intended for checking relief valves without removing them from the pressurized apparatus. Examples of known testing apparatus of this type are shown in the following U.S. patents: Trevisan U.S. Pat. No. 4,428,223, Thompson U.S. Pat. No. 4,349,885 and Thompson U.S. Pat. No. 4,761,999. These and other prior art devices, while able to produce the desired set pressure testing, suffer from being awkward and unwieldy to use. In particular they are difficult to set up on a valve and they do not provide a ready means for calibrating their transducers.
To perform a test the apparatus is mounted on the valve, which, depending on the valve construction, may require removal of a cap and handle. The test apparatus has a clamp to grab the valve bonnet, with a lifting cylinder and transducer disposed on the clamp. The lifting cylinder applies an opening force to the valve which force is measured by the transducer. Together with knowledge of the pressure in the apparatus and the valve seat dimension, the set pressure can be determined from the force measurement.
Since the test apparatus must accommodate valves ranging in size from a half inch to eight inches, the clamp has to be adjustable. This is typically accomplished by splitting the clamp into two halves which are movable toward and away from each other as needed.
The required adjustability of the clamp complicates mounting of the lifting cylinder on the clamp because a rigid structure cannot be used; a rigid support would defeat adjustability. One way around this is to mount to only one half of the clamp. The resulting structure is flimsy and difficult to use because a test may require several successive mounting and unmountings of the test device.
Regarding this last point, consider how the usual valve test is performed. If the set pressure of the valve is not at the desired point, the test apparatus must be removed to permit access to the valve's adjustment nut. The nut is moved as required to adjust the set pressure to the design point. Then the test apparatus is remounted to check the new set point. This checking and adjusting process continues until the design set point is reached.
Another factor in the ease of use of a testing apparatus is the ability to check the accuracy of the transducer. Prior art devices require removal of the apparatus from the valve and putting it on a special fixture to calibrate the transducer.